1. Technical Field
The present invention relates to a cyanide-free electroplating solution for depositing monovalent copper, silver, gold and other metals.
2. Background Art
Copper plating has been successfully accomplished for many years from cyanide-based plating solutions. In these solutions, copper is present as the monovalent copper cyanide complex. The solution may also contain free or uncomplexed alkali cyanide, alkali hydroxide and complexing agents such as alkali-tartrate to help dissolve copper anodes. Although these solutions have been successful, the industry has constantly been in search of a substitute for the poisonous cyanide ions.
Acidic divalent copper solutions have been commercially successful and are free of cyanide; however, these divalent solutions require twice as much total current to deposit the same amount of copper as do monovalent copper solutions. Furthermore, the acidic solutions cannot achieve the proper adhesion of copper when plating directly onto steel.
Alkaline divalent copper solutions have achieved limited commercial acceptance. These solutions are free of cyanide and are capable of plating directly onto steel with good adhesion. Since copper is divalent, however, the plating rate for a given amount of current is the same as that obtained from acidic divalent copper solutions.
To date, there are no commercially successful monovalent copper plating baths that are free of cyanide, stable, and capable of plating directly onto steel with good adhesion.
Cuprous halides with excess alkali halides have been proposed in the form of chlorides or iodides. Neither of these have reached commercial acceptance.
U.S. Pat. No. 1,969,553 describes a process for plating monovalent copper based on a solution containing sodium thiosulfate and cuprous chloride. This process was studied further and reported at the 77th general meeting of the Electrochemical Society, Apr. 26, 1940. A more recent study of the cuprous thiosulfate bath was reported in May 1981 at the annual technical conference of the Institute for Metal Finishing at Herrogate, U.K. These baths plated copper from monovalent solutions in which copper was complexed with a thiosulfate ion. It was reported that the stability of the bath was further improved with the addition of a sulfite ion. The pH of the solutions was in the range of 6 to 11, with the optimum range being 8.5 to 9.5. Acidic solutions having a pH of 6 or less were reported to be unstable and sulfur dioxide, resulting from the acidified sulfite ion, continuously evolved from these solutions. The authors concluded that these plating baths offered no significant improvement over an alkaline cupric pyrophosphate bath, and no further work has been reported to date involving thiosulfate-based monovalent copper plating baths.
Silver has been deposited commercially from cyanide baths for many years with good success. These silver solutions contain silver complexed with the cyanide ion, additives to improve the deposit or the plating bath, and may contain free alkali cyanide. The industry has also shown a strong interest in cyanide-free silver plating baths because of the toxic nature of the cyanide ion.
U.S. Pat. No. 4,126,524 discloses a cyanide-free silver plating bath in which silver is complexed with imides of organic dicarboxylic acids. This process has achieved some commercial success, however, the organic compounds are expensive and some instability has been reported.
Cyanide-free silver baths have also been reported based upon silver sulfite and silver thiocyanate complexes. However, these solutions have not achieved commercial success at this time.
Silver has been deposited from cyanide-free solutions based upon the thiosulfate ion. Such baths are disclosed in U.S. Pat. Nos. 3,984,292 and 4,067,784. Also, F. I. Kukov et al. (R. Zh. Korr. i Zasch. of Korr. 12K460, 1990) discloses a silver plating bath based upon thiosulfate, sulfite, and thiocyanate at a pH of 4 to 6. None of these solutions, however, have achieved commercial acceptance at this time.
Gold plating baths are also preferably cyanide-free and such baths have been disclosed based upon sulfite, thiocyanate, and thiosulfate complexes. These gold baths are mainly alkaline and have achieved some limited success in spite of the fact that they are not as stable as gold baths based upon the cyanide ion.